1. Field of the Invention
This invention relates to a vibration type motor in which a vibration member and a member which is in contact with the vibration member are moved relative to each other by vibration generated in the vibration member.
2. Related Background Art
FIG. 6 of the accompanying drawings shows the electrode arrangement of a piezo-electric element provided on the resilient member of a circular ring-shaped vibration wave driven motor according to the prior art. As shown, two groups of electrodes [A phase (A.sub.1 -A.sub.6) ] and B phase (B.sub.1 -B.sub.6) having a pitch of 1/2 of the wavelength of a vibration wave excited in a circular ring-shaped vibration member are spatially disposed with a pitch 1/2, i.e., with a phase deviation of 1/4 of said wavelength, and a sensor electrode S for detecting the vibrated state of the circular ring-shaped vibration member, i.e., an electrode such as a piezo-electric element generating an electromotive voltage by the vibration of the vibration member, and common electrodes C (C.sub.1, C.sub.2 and C.sub.3) are further provided between these two groups of electrodes. The vibration member comprises a resilient member of a metal or like material and a piezo-electric element (e.g. PZT) as an electro-mechanical energy conversion element attached to the resilient member, and the associated electrode of the electro-mechanical energy conversion element which is adjacent to the resilient member is short-circuited through the metallic resilient member. The common electrodes C.sub.1, C.sub.2 and C.sub.3 are rendered conductive with the resilient member by an electrically conductive paint or the like, and the potential difference from the resilient member is zero. When a potential of a certain magnitude relative to the potential of the common electrodes C is input to driving electrodes A.sub.1 -A.sub.6 and B.sub.1 -B.sub.6, a potential difference is produced between the front and back of the electro-mechanical energy conversion element and strain is created. Signs - and + shown on the electrodes indicate the directions of polarization of the piezo-electric elements in the electrode portions.
By a voltage V=Vo sin .omega.t being applied to the group of A phase electrodes A.sub.1 -A.sub.6 and a voltage V=Vo sin (.omega.t.+-..pi./2) being applied to the group of B phase electrodes B.sub.1 -B.sub.6, a travelling vibration wave is generated in the resilient member and a conventional movable member (not shown) such as a rotor which is in pressure contact with the resilient member is moved by friction. Also, by changing the signs (+) and (-) in the aforementioned equation, i.e., advancing or delaying the phase of one of the applied voltages with respect to that of the other, it is possible to change the direction of movement of the movable member, and in the case of (-), i.e., where a frequency voltage delayed in phase by .pi./2 (90.degree.) with respect to the frequency voltage applied to the A phase is applied to the B phase, the movable member is rotated clockwise (CW), and in the case of (+), i.e., where a frequency voltage advanced in phase by .pi./2 (90.degree.) with respect to the frequency voltage applied to the A phase is applied to the B phase, the movable member is rotated counter-clockwise (CCW).
A signal detected from the sensor electrode S is a signal having a certain phase relation with the frequency voltage applied to the A phase when the movable member is in a resonance state, but it becomes a signal deviating from said certain phase relation with the frequency voltage applied to the A phase when the movable member is in a non-resonance state. So, by detecting the phase difference between the driving voltage and the detection signal, it is possible to know whether the vibration is in a resonance state of great amplitude or how much the vibration deviates from a resonance state, and by determining from this information the frequency to be applied, the control of the rotational speed is possible.
FIG. 7A of the accompanying drawings is a fragmentary cross-sectional view of the resilient member of the vibration wave driven motor. A piezo-electric element 2 as an electro-mechanical energy conversion element is attached to the resilient member 1 of FIG. 7A. Symbols below the element 2 represent the positions of the electrodes of FIG. 6. A number of grooves each having a width t.sub.1 and a depth h.sub.1 are formed in the surface of contact of the resilient member with the rotor (not shown) over the entire circumference thereof. Some of these grooves are deep grooves of a depth h.sub.2 to prevent the creation of noise.
The spacing between the deep grooves of FIG. 7A may be suitably selected, but if this spacing is 60.degree., the vibration of the 3rd-order mode can be suppressed. As described above, the arrangement of the deep grooves gives birth to the effect of suppressing a single or a number of modes of noise and thus preventing noise.
At this time, the electrode pattern of the piezo-electric element which is an electro-mechanical energy conversion element and the pattern of said deep grooves are arbitrarily positioned.
In the case of the combination of the sensor electrodes S and the deep groove pattern as shown in FIG. 7A, the deep groove portion is near the sensor electrode S, and as shown in FIG. 7B of the accompanying drawings, the rigidity of the portion of the sensor electrode S varies. At this time, there is obtained the relation of the frequency f vs. the phase difference .theta..sub.A-S between the A phase and S phase as shown in FIG. 8 of the accompanying drawings. When the frequency of the resilient member is a resonance frequency, the phase difference .theta..sub.A-S is -135.degree. for CW and -45.degree. for CCW.
However, since the rigidity of the portion near the sensor electrode S varies as shown in FIG. 8, the amplitude and wavelength of the travelling vibration wave change delicately, and the CW rotation and the CCW rotation deviate from each other in opposite directions with respect to the ideal f vs. .theta..sub.A-S curve (the portion indicated by dotted line in FIG. 8).
Consequently, there is the problem that a right resonance frequency is not detected even if the .theta..sub.A-S phase difference is adjusted to -135.degree. (for CCW, -45.degree.). That is, in the example of the prior art described above, if the positional relation between the electrode pattern of the piezo-electric element which is an electro-mechanical energy conversion element and the deep groove pattern of the resilient member is arbitrary, the portion in which the rigidity of the deep groove portion varies is disposed near the sensor electrode S as shown in FIG. 7B, and this adversely affects the signal of the sensor electrode S and thus, a difference occurs relative to the true f vs. .theta..sub.A-S curve, as shown in FIG. 8. That is, near the deep grooves, strain differs and the center of the electrode for a sensor becomes as if it deviated and thus, it follows that a right resonance frequency is not detected.